ANNO ACCADEMICO 2017/2018
UNIVERSITÀ DI PISA
Facoltà di Scienze Matematiche, Fisiche e Naturali
Dipartimento di Chimica e Chimica Industriale
Corso di Laurea Magistrale in
“Chimica Industriale”
High yield production of few-layers graphene for
polymer composite applications
Relatori: Controrelatore:
Dr. Vittorio Pellegrini Prof. Luca Labella
Prof. Andrea Pucci
Candidata:
Abstract
Graphene is a two-dimensional (2D) material consisting of a monoatomic layer of carbon atoms arranged in a hexagonal structure. Its remarkable properties make it an ideal material for many applications: one of the most promising is its use as filler in composites in order to enhance mechanical, thermal, electrical and gas barrier properties of pristine polymers. Many studies report significant improvements of the above-mentioned properties due to the incorporation of graphene in the polymer matrix. However, these works focused on lab-scale approaches, thus leading to a lack of reliable data between laboratory research and industrial production. Therefore, there is need for further data to fill this gap, which motivates the present work. For this reason, the properties of graphene-based polymer composites produced by scalable process are investigated in the present work. In addition, the study focuses on the effect that the aspect ratio of graphene flakes (i.e. the ratio between the lateral size and thickness) has on the mechanical and gas barrier properties of the produced composites.
The thesis is organized as follows:
The first part focuses on the production and characterization of graphene flakes with different aspect ratio exploiting the wet-jet mill (WJM) technique, a large-scale production method developed and patented by the Graphene Labs researchers. This method consists on a liquid-phase exfoliation (LPE) of graphite by taking advantage of high shear forces enabling to separate the graphite layers, thus obtaining few-layer graphene (FLG). The main advantages of the WJM technique are the production of the high concentrated graphene-solvent inks and high production rate with 100% yield of graphene compared to other techniques, which make it a scalable method.
The second part reports the impact of graphene flakes incorporated in acrylonitrile-butadiene-styrene (ABS) and linear low-density polyethylene (LLDPE) matrices employing three different methods, i.e. laboratory scale, semi-scalable and scalable, in order to investigate the effect of different production process on the composites mechanical properties. The influence of graphene flakes aspect ratio on the tensile and gas-barrier properties of the composites is also investigated. To this end much of the attention is devoted to the scalable method used to produce composites by adding graphene of different aspect ratio using first an impregnation
process of polymer on powder or pellets form followed by melt extrusion and injection molding. The extrusion and injection molding are the manufactory methods mostly used in polymer industry, thus making this process potentially scalable. Moreover, the extruded pellets are used to produce films to study the oxygen permeation of the polymer composites having graphene flakes with different aspect ratios. Lastly, the effect of graphene on thermal behavior and crystallinity of the polymers are studied by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques.
The experimental research activity presented in this thesis work has been carried out in the Graphene Labs of the Istituto Italiano di Tecnologia (IIT) in Genova.
CONTENTS
1. INTRODUCTION ... 1
1.1 GRAPHENE: A PROMISING MATERIAL ... 4
1.1.1 Properties of graphene ... 5
1.2 PRODUCTION METHODS OF GRAPHENE ... 7
1.2.1 Bottom-up techniques ... 8
1.2.2 Top-down techniques ... 10
1.3 GRAPHITE EXFOLIATION BY WET-JET MILLING ... 15
1.4 APPLICATIONS OF GRAPHENE ... 16
1.5 GRAPHENE-BASED POLYMER COMPOSITES ... 18
1.5.1 Mechanical reinforcement ... 19
1.5.2 Gas barrier properties ... 20
1.5.3 ABS and LLDPE polymers ... 22
1.6 GRAPHENE-POLYMER COMPOSITE PRODUCTION ... 24
1.6.1 In-situ polymerization ... 24
1.6.2 Solution mixing ... 24
1.6.3 Melt blending ... 25
1.7 AIM OF THE STUDY ... 26
2. EXPERIMENTAL PART ... 28
2.2 WET-JET MILLING (WJM) ... 28
2.3 GRAPHENE POWDER PRODUCTION ... 30
2.4 COMPOSITE PRODUCTION ... 31 2.4.1 Impregnation ... 31 2.4.2 Hot-press ... 32 2.4.3 Extrusion ... 32 2.4.4 Injection molding ... 35 2.5 CHARACTERIZATION TECHNIQUES ... 37
2.5.1 Atomic force microscopy (AFM) ... 37
2.5.2 Transmission electron microscopy (TEM) ... 37
2.5.3 Scanning electron microscopy (SEM) ... 38
2.5.4 X-Ray diffraction (XRD) ... 39
2.5.5 Raman spectroscopy ... 40
2.5.6 Thermogravimetric analysis (TGA) ... 41
2.5.7 Differential scanning calorimetry (DSC) ... 42
2.5.9 Oxygen permeation (OP) ... 45
3. RESULTS AND DISCUSSION ... 47
3.1 PRODUCTION OF GRAPHENE WITH DIFFERENT ASPECT RATIO ... 47
3.1.1 Morphological analysis of graphene dispersion in NMP ... 49
3.2 GRAPHENE-POLYMER COMPOSITES ... 58
3.2.1 Laboratory scale, semi-scalable and scalable production... 58
3.2.2 Effect of aspect ratio on tensile properties ... 64
3.2.3 Effect of graphene flakes aspect ratio on gas barrier properties ... 74
3.2.4 Further characterizations on graphene-polymer composites ... 80
4. CONCLUSION AND FUTURE PROSPECTIVES ... 87
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1. INTRODUCTION
Research in new materials has been fundamental for the progress of our civilization. In fact, some periods are defined with the predominant material used, i.e. “Stone Age”, “Bronze Age” and “Iron Age”. The progress in materials continues to be one of the most important factors and objective for the scientific and industrial development.
Nowadays, thanks to their versatility, plastics have become key materials in many sectors such as packaging, construction, transportation, renewable energy, medical devices or even sports (see Figure 1). The world production of plastic materials in 2016 was about 280 million tons [1].
Figure 1. Examples of plastics applications in everyday life [1].
However, some polymers have drawbacks that limit their use in plastic products, such as low mechanical and gas barrier properties and low thermal stability. These problems can be solved by the incorporation of reinforcing materials in the polymer matrix, thus obtaining polymer composites.
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Composite materials are currently used in a wide range of applications, for example in automotive and aerospace industries, but also in the marine field, wind energy generation (composites for wind turbines blades [2]), to cite a few. The most common used fillers in industry are calcium carbonate (CaCO3), talc, glass fibers and carbon fibers [3].
These fillers can improve polymers properties, such as stiffness, toughness and gas barrier, but the achievement of a significant improvement in the composite properties often requires incorporation of a large amount of the filler in the materials, which imparts drawbacks to the composite such as brittleness or opacity. For example glass and carbon fiber are used between 8 and 25 vol% in polypropylene [4], talc and CaCO3 from 10 to 40 wt% in ABS [5].
Polymer nanocomposites are a new class of materials in which the fillers have at least one dimension in the nanometer scale. The reduction of size obtained in such materials increases the specific surface area of the filler, providing larger matrix-filler interface and so more mutual interactions. As a result, large reinforcing effect may be reached at much lower filler content when compared to classical composites. However, the shape of filler is an important factor as it affects the properties of composites and it can be described in terms of “aspect ratio” (α) which is defined as the ratio of length to diameter for a fiber, or the ratio of length to thickness for platelets and flakes [3]. For spheres, which have minimal reinforcing capacity, the aspect ratio is unity [5]. In developing reinforcing fillers, the aims are to increase the aspect ratio of the particles and to improve their compatibility and interfacial adhesion with polymer matrix in order to enhance and optimize the primary function of the filler, such as mechanical property modifier.
Table 1. Shape and aspect ratio of different fillers.
Shape Aspect ratio Example
Sphere 1* CaCO3
Platelet 4-30* Talc
Fiber 20-200* Glass fiber, carbon fiber, carbon nanotube Flake 50-200* Mica, graphite
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In recent years, graphene started to be considered a novel material with a great potential as filler for polymers due to its two-dimensional nature and peculiar properties. In fact, graphene offers an exciting blend of mechanical, electrical and thermal properties because of its unique two-dimensional honeycomb crystal structure and ultra-strong sp2 carbon bonding network. In particular, the elastic modulus of an individual graphene sheet is ~ 1 TPa [6], its value of in-plane thermal conductivity is above 3000 Wm-1K-1 [7] and it has a
high charge carrier mobility of ~2.5×105 cm2 V-1 s-1 [8]. Moreover, graphene has a great
specific surface area (~2630 m2 g–1 [9]), which combined with its other exceptional
properties confers to this material the great potential to significantly increase polymer composite properties even in low concentrations. Many fundamental studies have shown that incorporation of graphene into polymers leads to enhanced mechanical [10], thermal [10][11], and electrical [12] properties, paving a way for the development of high strength, lightweight polymer composites and their applications in many fields, i.e. automotive, aerospace, electronics, etc.
Nanocomposites based on graphene have been developed with a wide range of polymers. For example, Yan Li et al. [13] and Benhui Fan et al. [14] reported the study of polystyrene (PS) composites with graphene nanoplatelets (GNP) while Brian M. Cromer et al [15] investigated the reinforcement effect of GNP in polypropylene (PP) matrix. Many studied are also reported on polyethylene (PE) filled with GNP [16][17][10] or functionalized graphene [18] and on acrylonitrile-butadiene-styrene (ABS) filled with GNP [19][20] or few layers graphene (FLG) [21]. However, all these works reported studies on composites produced in laboratory scale. On the contrary, the present work will focus on scalable approaches both for the production graphene, using wet-jet mill (WJM), and for the production of graphene-based polymer composites, by melt extrusion. Moreover, we will exploit graphene flakes with tuned morphology (lateral dimensions and number of layers) in order to study the impact of flake aspect ratio on mechanical and gas barrier properties of the produced composites. In fact, graphene with a higher aspect ratio is expected to lead to a greater improvement in the composite properties allowing to reach the required composite performance at a lower amount of filler in the final product.
Here we will focus on two polymers: linear-low-density-polyethylene (LLDPE) and acrylonitrile-butadiene-styrene (ABS) both provided by ENI VERSALIS.
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1.1 Graphene: a promising material
The progress in science and technology permits the discovery, study and production of new advanced materials. Nowadays, nanotechnology acquires a key role in the field of innovative products.
Nanotechnology concerns design, production, characterization, and application of structures with controlled shape at the nanometer scale, generally from 1 to 100 nm. In particular, nanomaterials with only one side in the nanoscale are referred to as a two-dimensional (2D), if two sides are of nanometer length, the structure is referred to as a one-dimensional (1D), and if all the three dimensions are confined in the nanoscale is referred to as a zero-dimensional (0D).
Size reduction of materials to nanometer scale leads to the increase in surface area to volume ratio, which makes surface phenomena more important compared to the bulk ones. This feature provides different mechanical, thermal and catalytic properties to material, enabling to obtain innovative tools and technologies such as nanoparticles based cancer therapy [22], high performance composites [17][11][10], electronic applications [23][24], sensors and biosensors [25].
In recent years, graphene has obtained enormous interest as a new material. Graphene is a two-dimensional material made of sp2-hybridized carbon atoms arranged in a hexagonal lattice. It is the building block of graphite which is composed by many layers of graphene held together by van der Waals forces with inter-layer distance of 3.35 Å [26] (see Figure 2b). Because of this layered structure, it has been tried for a long time to split such materials into individual atomic layers, even if it was unclear whether free-standing atomic layers could exist because thin films become thermodynamically unstable below typically few dozens of layers [27]. Although graphite was used since many years in pencils and other applications [28], graphene was first time isolated in 2004 by Prof. Andre Geim and Prof. Konstantin Novoselov at the Manchester University [29] by mechanical exfoliation of graphite utilizing scotch tape.
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Figure 2. Structure of a) graphite, b) graphene, c) nanotube d) and fullerene. Adapted from [30].
Graphene completes the set of carbon materials with different dimensionalities that can be considered to derive from graphite: 1D carbon nanotubes (CNTs, Figure 2c) [31], which have a cylindrical morphology; 0D fullerenes [31][32], which have the form of a hallow sphere and are composed by carbon atoms organized in pentagonal and hexagonal rings that confer the curvature to the structure as shown in Figure 2d.
1.1.1 Properties of graphene
Graphene has remarkable properties, which make it an ideal material for many applications such as filler in polymer composites [20][17], electrodes in super capacitor [33] and active material for sensors [25][34]. Some of the most important graphene properties are listed in the following:
Room temperature electron mobility of 2.5×105 cm2 V-1 s-1 [8];
Young’s modulus of ~1 TPa [6];
Intrinsic strength of ~130 GPa [6];
Thermal conductivity above 3000 Wm-1K-1 [7];
Impermeability to any gases [35];
High surface area (theoretically predicted ~2630 m2 g-1) [9];
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However, these characteristics are achieved only for the highest-quality monolayer defect-free graphene [37]. The quality of graphene (i.e. the absence of defects in its structure) and the number of layers by which it is composed depends on the production process. Graphene can be produced via growth techniques (such as chemical vapour deposition or thermal sublimation of silicon carbide) [31][39] or starting from graphite via exfoliation techniques (such as micromechanical cleavage and liquid-phase exfoliation) [40][41]. Growth techniques allow to produce high quality material, but they are expensive and not easily scalable. In case of CVD graphene, transfer approaches from the growth copper substrate to the application substrate are still a challenge [42][43].
In recent years, in addition to single-layer graphene (SLG) grown by CVD or on SIC, graphene nanoplates (GNP, graphite material with thickness <100 nm [44]), multilayer graphene (MLG, 2-10 layers [44]) and few layer graphene (FLG, 2-5 layers [44]), have been extensively produced and studied and got significant commercial interest mainly as fillers in polymer composites. For the production of such different named graphene various methods are used. However, some of them are not scalable and few methods raised the graphene’s quality issue. Therefore, for industrial exploitation of graphene, it is very important to develop a scalable production method which can fulfill the requirements of the polymer composite applications. The graphene used in this thesis work is produced by exploiting the liquid-phase exfoliation, which can be scalable and the resulting graphene flakes are defect-free and stacked in a few-layer configuration. In the following section, some of the main graphene production methods are discussed.
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1.2 Production methods of graphene
Graphene can be produced by two different synthesis routes: bottom-up and the top-down. Bottom-up synthesis techniques start from molecules as raw material, which are arranged to form the graphene structure. Amongst them, the most important methods are:
Epitaxial growth on silicon carbide
Chemical vapor deposition (CVD)
In contrast, top-down approach consists of exfoliating bulk graphite to obtain graphene. The inter-layer van der Waals interaction energy is about 2 eVnm-2 and the order of magnitude of the force required to exfoliate graphite is about 300 nNµm-2 [45]. This weak force can be achieved with different approaches such as:
Micro mechanical cleavage (MC)
Liquid phase exfoliation (LPE)
Figure 3 summarized the above-mentioned production methods of graphene.
Figure 3. Schematic illustration of the most important graphene production methods. Adapted from [46] and [34].
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1.2.1 Bottom-up techniques
1.2.1.1 Epitaxial grown on silicon carbide
It has been demonstrated that graphene layers can be grown either on the silicon or carbon faces of a silicon carbide (SiC) wafer. When it is annealed at high temperature (about 1300 °C) [32] under ultra-high vacuum (UHV), the sublimation of silicon atoms, leaves a graphitized surface. In fact, as Si sublimates, the carbon atoms rearrange themselves in a graphene-like hexagonal structure [46].
Figure 4. Schematic representation of graphene grown on SiC. Adapted from [46].
The quality of such graphene can be very high, with lateral size up to hundreds of micrometres [39]. A considerable advantage of this method is that insulating SiC substrates can be used so that the transfer of graphene to another insulator is not required. However, the two major drawbacks of this method are the high cost of SiC wafers and the high annealing temperatures used [34]. As a result of high-temperature growth, high substrate cost and small-diameter wafers, the use of SiC-grown graphene will probably be limited to electronic applications such as high-frequency transistors [47]. Moreover, the large-scale structural quality is limited at present by the lack of continuity and uniformity of the grown film. Regions of different film thicknesses are produced and such inhomogeneity do not meet the demand of large-scale device production because the electronic structure of the film depends strongly on the number of layers. For example, although monolayer graphene is a gapless semiconductor, an energy gap ~30 meV is present in bilayer graphene under an electric field [48].
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1.2.1.2 Chemical vapor deposition (CVD)
Figure 5. Schematic representation of CVD furnace
Chemical vapor deposition process allows growing graphene films by pyrolysis of a gaseous precursor on a metal catalyst substrate, typically copper or nickel. The growth mechanism can be described as follows [31]:
i) Catalytic decomposition of gaseous source;
ii) Nucleation on the catalytic substrate;
iii) Growth of nuclei to form graphene islands and full substrate coverage by graphene.
The parameters that is possible to modify in order to optimize the conditions of growth are the choice of precursor, substrate, pressure, and temperature.
Precursor: Hydrocarbons sources, such as methane (CH4), are used as precursors to
produce graphene. Moreover, the addition of hydrogen (H2)can help to control the
structure, morphology, and size of graphene [49][50] and the purification of the catalyst from the carbon residues.
Substrate: The substrate is the material on which graphene grows in the CVD process but it also provides other functions such as catalytic surface. Nickel or copper substrates are usually used for multilayer and monolayer graphene growth.
Pressure: The pressure can be optimized in order to have low concentration and high velocity of the precursors mass feed in order to make the reaction more controllable. Generally, it can vary from few atmosphere to vacuum [51][50][38].
Temperature: High quality products can usually be obtained at relatively high temperatures (generally the temperature used is above 1000 °C) [46].
In addition to thermal CVD where heat leads to breakage chemical bonds in the precursors and promotes the aforementioned reactions, there are other methods to achieve this purpose,
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such as plasma, light, or laser. Among these modified CVD methods, plasma-enhanced CVD (PECVD) is a powerful tool to enable many film deposition processes that are very difficult to achieve by solely adjusting temperature in a typical thermal CVD reaction. These methods have great potential in achieving high quality 2D films at moderate temperatures, which is important for growth on polymeric or glass substrates [50].
The CVD is the most used bottom-up technique, providing high quality graphene, in terms of large grain size (up to hundreds of micrometers) and controllable number of layers [50][51]. The growth of large-domain single crystalline graphene with controllable number of layers is of central importance to ensure uniformity and remarkable electronic properties for large-scale integration of graphene in devices. CVD process allows the growth of uniform graphene films whose size is limited only by the size of the substrate and the growth system [38]. Literature reports the production of giant single crystals of monolayer graphene with a lateral size up to 5 mm and bilayer graphene with the lateral size up to 300 mm [51]. However, the high production cost of CVD makes it suitable only for high value added application, i.e. electronics [50].
1.2.2 Top-down techniques
1.2.2.1 Micromechanical cleavage (MC)
Figure 6. Micromechanical exfoliation of graphene. a) Adhesive tape is pressed against the bulk graphite material so that b) the top few layers are attached to the tape; c) The tape with crystals of layered material is
pressed against a surface of choice. Upon peeling off, d) the bottom layer is left on the substrate. Adapted from [52].
The micromechanical cleavage (often referred to as a scotch-tape technique) is the method used to isolate graphene for the first time.
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MC consists in the consecutively peeling off of graphite using adhesive tape (see Figure 6) and provides crystals of high structural and electronic quality which can reach millimeter size [40]. However, this technique is useful only for fundamental studies and it is not suitable for a large-scale production, due to time and implementation cost constrains [40].
1.2.2.2 Liquid-phase exfoliation (LPE)
Liquid-phase exfoliation is an important approach to produce large quantities of graphene because it is versatile and potentially up-scalable for producing industrially-relevant quantities. This method can be also used to produce others 2D materials in addition to graphene (such as transition metal dichalcogenides (TMDs) and phosphorene [53]) using a wide range of solvents. In bulk graphite, the van der Waals attractions between adjacent layers are strong enough to make the complete exfoliation into individual layers difficult. In order to exfoliate successfully these layered materials it is necessary to overcome the attractions between the adjacent layers.
There are several methods to provide the energy needed to separate the layers such as application ultrasound [54][55] or high shear forces [56] in appropriate solvents. Moreover, other techniques such as ball milling [57][58] or approaches based on exploiting hydrodynamic apparatus [59][60] can be used as alternative strategies.
Generally, LPE involves three steps:
i) dispersion of graphite in a suitable solvent;
ii) exfoliation;
iii) purification.
Figure 7. Liquid-phase exfoliation of graphene: a) graphite, b) chemical wet dispersion , c) ultra-sonication and d) final dispersion after the ultracentrifugation process [61].
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In the first step, the bulk material is immersed in a suitable solvent or aqueous surfactant solution and then subjected to ultrasonication or shear forces. The interactions at the liquid-nanosheet interface reduce the net exfoliation energy and stabilize the graphene layers against aggregation [41]. The weak interlayer attractions need to be outmatched by imparting energy during the exfoliation process. A purification step is often required because the as-produced dispersion contains nanosheets with different lateral sizes and thicknesses. For this reason, after exfoliation, the dispersion can be subjected to centrifugation in order to remove un-exfoliated material and perform size selection. In general, these methods result in 2D nanosheets with lateral sizes in the range 100 nm-100 μm and thicknesses can reach one or few layers [53].
However, all these techniques have drawbacks. In particular, the main limitations of LPE are the low production rate (g h-1) and concentration of the obtained dispersion. For example the ultra-sonication technique requires from 6 to 90 hours of processing time [62], and the dispersion concentration is in the order of a few g L-1 [63][64]. Whereas, higher production rates are achieved with the micro-fluidization (4-9 g h-1), if compared with sonication [60] and concentration of multilayer flakes which can be achieved is in 50-100 g L-1 range with the aid of surfactants [60] but their presence is problematic for biological or electronic applications [26]. In fact, it is difficult to completely remove the surfactant from the nanosheet surface after processing which can potentially change the graphene properties.
In order to understand which solvent is more suitable to help the exfoliation of graphite, it is useful to consider the thermodynamics of the process. Thermodynamically spontaneous processes require the Gibbs free energy involved in the process to be negative (∆𝐺 < 0). In particular, to exfoliate and stabilize a 2D crystal in a solvent, the Gibbs free energy of the mixture solvent-layered material (∆𝐺𝑚𝑖𝑥) should be minimized:
∆𝐺𝑚𝑖𝑥 = ∆𝐻𝑚𝑖𝑥− 𝑇∆𝑆𝑚𝑖𝑥 , (1) where, ∆Hmix and ∆Smix are the change in enthalpy and entropy involved in the mixing
process, respectively. Being ∆Smix small for the dispersion of graphene in a solvent due to its
large size and considerable rigidity compared to molecules [65], ∆Hmix must be minimized
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The enthalpy of mixing can be approximately calculated by the following relation [66]:
∆𝐻𝑚𝑖𝑥 𝑉𝑚𝑖𝑥 ≈ 2 𝑡𝐺(√𝐸𝑆𝑢𝑟 𝐺 − √𝐸 𝑆𝑢𝑟𝑆 ) 2 𝜑𝐺 , (2) where, 𝐸𝑆𝑢𝑟𝐺 and 𝐸
𝑆𝑢𝑟𝑆 are the surface energy of graphene and solvent respectively, 𝑡𝐺 is the
thickness of a graphene flake and 𝜑𝐺 is the graphene volume fraction.
From equation (2), solvents whose surface energies match with the one of graphene (~ 68 mJ m-2) [41] are expected to minimize the energy of exfoliation since they minimize
the interfacial tension between solvent and graphene [26][67][68]. For graphite, surface energy is defined as the energy per unit area required to overcome the van der Waals interaction between the graphene layers [63].
However, generally literature reports only the surface tension (γ) for solvents, which is related to the surface energy by the equation (3) [69][63]:
𝛾 = 𝐸𝑆𝑢𝑟𝑆 − 𝑇𝑆𝑆𝑢𝑟𝑆 , (3) where 𝐸𝑆𝑢𝑟𝑆 is the solvent surface energy and 𝑆
𝑆𝑢𝑟𝑆 is the solvent surface entropy. It has been
proved that good solvents are those with room temperature surface tension γ ~40 mJ m-2,
which is equivalent to surface energy of ~68 mJ m-2 [41].
Although surface energy is an important parameter to understand the solvent-graphene interactions, another parameter which can be used is the Hildebrand parameter (δT) [70]. It
is defined as the square root of the cohesive energy density of the material:
𝛿𝑇 = √∆𝐻𝑣−𝑅𝑇
𝑉𝑚 , (4)
where, ∆𝐻𝑣 is the enthalpy of vaporization, R is the ideal gas constant, and 𝑉𝑚 the molar volume. Also in this case, it is expected that the best solvents for the dispersion have a Hildebrand parameter close to that of graphene (δT,G~23 MPa1/2) since [67]:
∆𝐻𝑚𝑖𝑥
𝑉𝑚𝑖𝑥 = 𝜑𝐺(1 − 𝜑𝐺)(𝛿𝑇,𝑆− 𝛿𝑇,𝐺)
2
, (5) where, 𝜑𝐺 is the graphene volume fraction, and 𝛿𝑇,𝑆 and 𝛿𝑇,𝐺 are the Hildebrand parameters of solvent and graphene, respectively.
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However, this parameter does not take into account dispersive (D), polar (P) and hydrogen-bonding (H) interactions[70], which are described by the Hansen parameters (δD,δP, δH ) in
which δT can be split [41]:
𝛿𝑇2 = 𝛿𝑑2+ 𝛿𝑝2+ 𝛿ℎ2 . (6) Solvents whose Hansen parameters match with the one of graphene are good solvent for the dispersion of graphene.
Despite the numerous theories developed, the theory based on the surface energy matching is generally the most used to identify which solvents are most suitable for graphite exfoliation. Unfortunately, the majority of suitable solvents, such as N-methyl-2-pyrrolidone (NMP, γ~40 mJ m-2) and N,N dimethylformamide (DMF, γ~37.1 mJ m-2) have some
disadvantages, for example NMP is an eye irritant and may be toxic to the reproductive organs [71], while DMF may have toxic effects on multiple organs [72]. Moreover, they have high boiling temperatures (NMP Tb=203 °C and DMF Tb=154 °C). Nevertheless, they
are extensively used in many industrial processes. On the other hand, most low boiling solvents, e.g. water, ethanol, and chloroform, have a surface tension unsuitable for the direct exfoliation of graphene (γ~72.8 mJ m-2, γ~22.1 mJ m-2 and γ~27.5 mJ m-2,
respectively) [26].Alternatively, stabilizers can be added, for example surfactants in an aqueous solution. In this case, the medium is environmentally friendly but it is difficult to remove the surfactant completely after the exfoliation process [26].
Despite these difficulties, LPE is a very promising scalable method for the production of graphene and for this reason it is used in this work. In particular, few-layers graphene were produced by LPE based wet-jet mill (WJM) process, developed and patented by IIT graphene labs [73][74], which will be described in the following paragraph.
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1.3 Graphite exfoliation by wet-jet milling
For large-scale applications, efficient and scalable methods to produce graphene is required. Although many production techniques have been developed [46] as described in paragraph 1.2, the most promising approach for large-scale production of graphene is the LPE of graphite [62].
In order to overcome the above-mentioned issues with graphite exfoliation (i.e. low production rate and concentration of the graphene dispersion), a different approach based on high-pressure wet-jet milling (Figure 8) was studied, optimized and implemented [73][74].
Figure 8. a) Photograph of the wet-jet mill machine used in this thesis work (D.Farina@Istituto Italiano di Tecnologia), schematic representation of b) WJM process and c) internal design of the nozzle used in the
WJM. Adapted from [74].
This technique allows to produce large quantities at high production rate (>20 g h-1 with concentration >10 gL-1) without the use of surfactants [74] and with 100% yield. In fact, the mixture of solvent (NMP) and graphite is pumped into the WJM chamber, which consists of nozzle where high pressure is applied, and the exfoliation is performed. There are four nozzles having different diameter (0.3, 0.2, 0.15 and 0.1 mm) used in the exfoliation process. The exfoliation of the flakes is carry out by the shear forces, the implosion of cavitation bubbles and the drastic pressure changes on the mixture of graphite and NMP.
At the end of the process, an ink of exfoliated graphite in NMP is collected and used for graphene characterization by means of transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Moreover, powdered graphene can be obtained using a freeze-drying process.
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1.4 Applications of graphene
Graphene has exceptional properties which enable its use in many application such as graphene-based electronics [75], energy generation and storage [76][77][33], sensor and composite materials [78].
Figure 9. Examples of graphene’s applications.
Graphene’s potential for electronics is justified by its high carrier mobility (~2.5×105 cm2 V-1 s-1[8]). Moreover it has a transmittance of 97.7% per layer [36] which
allows graphene to be used as transparent conductive coatings in electronic products such as touch screen displays. Graphene has also mechanical flexibility and chemical durability that are very important characteristics for flexible electronic devices [34].
Efforts are also concentrated on solar cells, where graphene can be used as the active medium or as a transparent electrode material, and in next-generation lithium-ion batteries where graphene acts as an advanced conductive filler in both cathodes and anodes [34][79].
Another application of graphene is for sensors production (for example to measure strain, gas environment, pressure and magnetic field) [34]. Chemically-functionalized graphene can be used for biomedical applications because it is capable of detecting a range of biological
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molecules including glucose, cholesterol, hemoglobin and DNA [25]. Nevertheless, before graphene can be used in the biomedical area, the study of its bio-distribution, biocompatibility and toxicity is important.
However, one of the most important and immediate application of graphene is its use as filler in polymers to enhance the mechanical, thermal, electrical and gas barrier properties. The production of graphene-based polymer composites requires graphene to be produced in large scale but also incorporated, and homogeneously distributed, into the matrices [34]. Thus, in the following paragraphs the field of graphene-based polymer composites will be introduced and briefly discussed.
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1.5 Graphene-based polymer composites
Over the past decades, polymers have replaced conventional materials (such as metals, ceramics, paper) in many applications due to their low manufacturing cost, great versatility and high specific strength [80]. Among all polymers, thermoplastics are the most used materials nowadays [3], which can be employ to produce plastic products useful in everyday life. For example, plastic parts are embodied in cars enabling to reduce costs and to have lower environmental impact due weight reduction and thus lower fuel consumption resulting in fewer CO2 emissions. Moreover, plastic packaging extend significantly the shelve life of
food reducing spoilage and waste. In addition, at the end of their life, plastics are still very valuable resources that can be transformed into new feedstock or into energy. In spite of these advantages, the main drawbacks of polymers are the low mechanical, gas barrier properties and thermal stability [81]. These problems promote the development of polymer composites [80][31].
A composite is a material obtained by mixing together two or more different components in order to get improved properties than those of the single constituents. In general, a composite material consists of:
A matrix, which is the component that holds the filler together to form the bulk of the material. It is often the largest component in the composite and transfers the external load to the filler.
A filler, which is the reinforcing material that gives its properties to the composite. The demand for polymer composites has increased in the last years due to the industrial request for high performance structural materials to be used in applications such as aircraft, automotive, military and sport facilities [81].
As already mentioned, the peculiar properties of graphene are appealing today for its use as a filler to form a polymer composite. There are many reports in the literature showing the improvements in the mechanical, thermal and gas barrier properties of the composites compared to the neat polymers [82]. These improvements depends on several parameters, such as distribution of graphene and interfacial interactions between graphene and polymer chains. Another important parameter is the morphology of graphene layers, which is also playing major role in the composite properties [81].
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1.5.1 Mechanical reinforcement
The impressive mechanical properties of graphene, i.e. Young’s modulus ~1 TPa and tensile strength ~130 GPa, are some of the reasons that makes it interesting as reinforcing agent in composites [31][78][83]. As representative recent works, Qaiser Waheed et al. [21] reported a increment about 70% and 43% in Young’s modulus and tensile strength, respectively of acrylonitrile-butadyene-styrene (ABS) due to the incorporation of 1 wt% of FLG in the polymer matrix. Furthermore, Karolina Gaska et al. [10] mentioned the Young’s modulus of low-density polyethylene (LDPE) is increased by 79% with 5 wt%. of GNP loading with respect to the pristine polymer while there was no change in tensile strength. In another study, P. Noorunnisa Khanam et al. investigated the effect of GNP in linear low-density polyethylene (LLDPE) matrix and found an increase in tensile strength of 47.3% at a loading of 4 wt%. Moreover, Pingan Song et al. prepared polypropylene (PP) composites filled with reduced graphene oxide (rGO) with increase in tensile strength and Young’s modulus up to 38% and 23% at 0.1 wt%, respectively.
However, the improvement in these polymer properties depends on various factors, such as type of polymer, type of functional moieties present on the surface of graphene to interact with the polymer chains and the composites preparation method [21][31].
In literature, various theoretical models have been proposed to predict and estimate the composite modulus as a function of matrix, filler, and interface properties. One of the simplest relationships is the ‘‘rule of mixtures” [84][85][83] which helps to predict the modulus of a composites, but it should always be considered as an approximation, since it assumes to have uniform distribution, perfect adhesion of the filler to the matrix and does not take into account aggregation of the filler at high contents [83].
Another important theoretical model is the “modified rule of mixtures” (MRoM) [83][86] which considers the orientation and geometry of the filler, which plays an important role in the prediction of composites Young modulus. A random orientation of fillers causes its reduction in modulus compared to aligned ones.
There are also important semi empirical models such as the Halpin-Tsai equation [83] where geometry and packing arrangement of the filler are considered. This model is the one that better describe the increase of Young's modulus with the increase in the amount of graphene and its aspect ratio.
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All these models will be described in more details in the “Results and discussion” section. In any case, each of them reports an increase in the Young’s modulus of the composite as the volume fraction and the aspect ratio of the filler increase.
1.5.2 Gas barrier properties
The gas permeability of polymers is an important physical property for many industrial applications [87][80]. For example, polymers with high barrier properties are required for food packaging applications in order to avoid the loss of flavor or quality due to polymers permeability to oxygen or other gases.
The high aspect ratio and very low gas permeability [35] of graphene make it a promising nanoscale fillers for gas barrier applications in polymer composites [88]. When graphene layers are homogeneously dispersed in a polymer matrix, it produces a tortuous path, which acts as a barrier for gases (see Figure 10). A high tortuosity leads to superior barrier properties and decreases the gas permeability of the graphene-polymer composites.
Figure 10. Schematic representation of the tortuous path of gas diffusion through a graphene-polymer composite.
There are several theoretical models [89][88] that allow to estimate the gas barrier properties of graphene-composites that consider different parameters of the filler that strongly affect the gas permeability, such as aspect ratio, dispersion and orientation of graphene layers. They describe the increase of the tortuosity of the gas diffusion path with volume fraction and aspect ratio of graphene.
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In Figure 11, the effect of different values of graphene aspect ratio (α) on the relative permeability (P/P0) of graphene-polymer composite is shown. It may be observed that the
gas barrier property of the composite significantly improves with the increase of graphene volume fraction (φ) and α.
Figure 11. Effect of graphene volume fraction (φ) and aspect ratio (α) of graphene layers on the relative permeability of composite [88].
For example, a reduction of carbon dioxide (CO2) permeability through a
graphene-polyethylene composite has been demonstrated [90]. In that work, the permeability of CO2 decreases with the increasing of filler content. In particular, the CO2 permeation
decreased by 34.7% for sample filled with 1 wt% of GNP respect to the pristine polymer. Another study reports the reduction of nitrogen (N2) permeability when thermoplastic
polyurethane (TPU) films are filled with graphitic platelets [12]. They found a 90% decreasing in N2 permeability with 3 wt% of filler.
Oxygen permeation (OP) is one of the most important properties in the packaging application to enhance the shelf life of any product. As shown in the literature the filler content and its morphology are the crucial factors in the permeability study. In this regard, the current thesis study emphasis on the OP of acrylonitrile-butadiene-styrene (ABS) and linear low-density polyethylene (LLDPE) filled with graphene produced by WJM. Importantly, the effect of filler morphology on the OP is studied at different loadings of graphene.
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1.5.3 ABS and LLDPE polymers
The polymer studied in this work are acrylonitrile-butadiene-styrene (ABS) and linear low-density polyethylene (LLDPE) as they are two of the most commonly used thermoplastics.
Acrylonitrile-butadiene-styrene is an amorphous and thermoplastic polymer, which consists of three monomers: acrylonitrile, butadiene, and styrene (Figure 12).
Figure 12. Monomers of the acrylonitrile-butadiene-styrene polymer.
Generally, the proportion of these three monomers is about 15-35% of acrylonitrile, 5-30% of butadiene and 40-60% of styrene. The result is a long chain of polybutadiene with shorter chains of poly-styrene-co-acrylonitrile. Variation in even one of the monomers relative proportion can drastically changes the mechanical and physical properties of this polymer.
Each of these three monomer gives particular properties to ABS: acrylonitrile provides chemical and thermal stability thanks to the interaction between the polar nitrile group, while butadiene, a rubbery material, increase toughness and impact strength, and styrene gives to the plastic a shiny, impervious surface.
The possibility to be injection molded and extruded make it useful in manufacturing various products such as musical instruments, automotive components, medical devices, kitchen appliances and toys, including Lego.
On the contrary, polyethylene is a semi-crystalline, thermoplastic polymer made from the monomer ethylene and it is the most common plastic used in packaging.
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Several types of polyethylene are produced and classified by density. Some examples of different kind of polyethylene are:
High-density polyethylene (HDPE)
Linear low-density polyethylene (LLDPE)
Low-density polyethylene (LDPE)
In semi-crystalline materials, like polyethylene, the properties of the material, such as density, depend on its crystallinity degree. HDPE is a polymer with linear chains that allow to increase its crystal packing compared with LDPE in which the long branches do not pack well. For this reason, HDPE has a higher density compered LDPE. Finally, LLDPE has shorter branches than LDPE, thus conferring to this polymer higher density compared to LDPE but still lower than HDPE. Notably, linear low-density polyethylene is the most used in food packaging applications since it offers offer superior strength and gas barrier properties compared to LDPE and lower rigidity than HDPE [91].
Table 2. Structure and density of HDPE, LDPE and LLDPE.
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1.6 Graphene-polymer composite production
The most important aim during the graphene-polymer production is to ensure a homogeneous dispersion of the filler in the polymer matrix in order to have an improvement in the polymers properties [83]. There are three main methods for the incorporation of graphene into polymer [93]:
In-situ polymerization;
Solution mixing;
Melt intercalation.
1.6.1 In-situ polymerization
During an in-situ polymerization, a monomer and graphene fillers are mixed together, in some cases in the presence of a catalyst [93]. After this step, the polymerization reaction is initiated by heat or radiation. If the fillers contain reactive functional group on the surface, covalent bond between fillers and polymers can be introduced. This allows the grafting of the filler on the polymer for the enhancement of compatibility between the components of the system, which enables the formation of a homogeneous dispersion and distribution of the filler in the polymer and so a good stress transfer between the matrix and the filler. One of the difficulties in this method is associated with the increase of viscosity during the polymerization process, that can limit the loading fraction of the graphene filler in the final composite [83].
1.6.2 Solution mixing
In this method, polymer and graphene are solubilized and dispersed in the same solvent. However, the finding of a suitable solvent, which can solubilize the polymer, disperse the graphene flakes and that can be completely removed from the final product [83] could be an issue. Moreover, aggregation of the graphene flakes could occurs during the solvent evaporation. To provide a better dispersibility in different solvents, functionalized graphene flakes are used. However, the functionalization can alter the properties of graphene.
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1.6.3 Melt blending
In the melt intercalation method, graphene is mixed with the polymer matrix in the molten state, meaning that no solvent is required. Homogeneous mixing of graphene and polymers is achieved by high shear mixing at elevated temperatures. This method has found many applications for the preparation of thermoplastic composites: thermoplastic polymers can be mechanically mixed with graphene at elevated temperatures using conventional industrial methods, such as extrusion and injection molding [82]. The high shear forces often required for an efficient mixing of the polymer with filler can cause defects and breakage of graphene sheets. Moreover, dispersity and distribution of the fillers in the matrix are worse compared with other methods (such as in situ polymerization and solution mixing) even if usually sufficient to confer good properties to the composites [83]. Nevertheless, this method is scalable considering that melt mixing is the procedure followed most widely in industry for the production of thermoplastic polymer composites.
Nowadays, the most studies on graphene-based polymer composites are reported with lab-scale methods, thus leaving a gap between laboratory scale and industrial process. This lack of available data on the properties of graphene-polymer composites obtained by scalable approach motivates the study of such materials produced by melt extrusion and injection molding compounding. In particular, Bandera® twin-screw extruder and Babyplast® injection molding machine, shown in the Figure 14, are used in this work.
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1.7 Aim of the study
Many studies reported the possibility to significantly enhance the mechanical and gas barrier properties of polymers by adding graphene in the matrix. However, these works are focused on lab-scale approaches, thus there is lack between laboratory research and industrial production . The need for further data to fill this gap motivates this thesis works to investigate the properties of graphene-based polymer composites produced by industrially scalable process, starting from the high yield wet-jet mill exfoliation of graphite to the melt extrusion compounding. Figure 15 summarizes the thesis workflow.
Figure 15. Aim of the Thesis work: production of graphene by WJM for scalable polymer composites manufacturing and study of aspect ratio effect on mechanical and gas barrier properties.
For the graphene production, the high-yield and scalable method based on the wet-jet mill, developed and patented in IIT Graphene Labs [73], is used. To prove that the final properties of composites can be affected by the production process, three different methods, i.e. laboratory scale, semi-scalable and scalable process, are considered for manufacturing graphene-polymer composites. The first method employs polymers impregnated with WJM-graphene to produce composite films obtained by hot-press. However, the low production rate and the non-reproducibility, make this process not scalable.
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If the impregnated polymer are extruded before being used to make films with the hot-press, the resulting material can be reproducible, giving a semi-scalable method limited only by the low production rate.
Finally, the employing of injection molding machine instead of the hot-press allows fast and reproducible production, compatible with industrial requirements.
Graphene with different aspect ratio will be produced by wet-jet milling and incorporated in acrylonitrile-butadiene-styrene (ABS) and linear low-density polyethylene (LLDPE) by the scalable extrusion and injection molding processes. In this way we will be able to evaluate the effect of the graphene’s aspect ratio on the mechanical and gas barrier properties of the produced composites.
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2. EXPERIMENTAL PART
2.1 Materials
Graphite: Sigma Aldrich, flakes, +100 mesh. Solvents:
N-Methyl-2-pyrrolidone (NMP): Sigma Aldrich, anhydrous 99.5%;
Dimethyl sulfoxide (DMSO): Sigma Aldrich, anhydrous, ≥99.9%;
Isopropyl alcohol (IPA): Sigma Aldrich, ≥99.8%;
Acetone: Sigma Aldrich, ≥99.5%.
Polymers:
Acrylonitrile butadiene styrene (ABS): ENI Versalis, Sinkral F332;
Linear low density polyethylene (LLDPE): ENI Versalis, Flexirene CL10U.
2.2 Wet-jet milling (WJM)
A mixture of graphite (~10 g) and NMP (1 L) was prepared and mixed with a mechanical stirrer at 100-120 RPM. The WJM (see Figure 16) exploits high pressure (180-250 MPa) to force the passage of the solvent/layered-crystal mixture through perforated disks, with adjustable hole diameters (0.3-0.1 mm) named nozzles, strongly enhancing the effectiveness of shear forces [73]. Importantly, the mechanical stirring was maintained during the whole WJM process.
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The WJM apparatus is schematized in Figure 17a. A hydraulic mechanism and a piston supplies the pressure (up to 250 MPa) in order to push the sample into a processor consisting of a set of 3 different perforated and interconnected disks: disk A, disk B and disk Ā (see Figure 17b).
Figure 17. a) Scheme of the wet-jet mill system, the arrows indicate the flow of the solvent through the WJM, b) Close-up view of the processor. The zoomed image in b) shows the channels configuration and the disks arrangement. The solvent flow is indicated by the white arrows. On the right side a top view of the holes and
channels on each disk is shown. Adapted from [74].
The disks A and Ā have two holes of 1 mm in diameter, separated by a distance of 2.3 mm from centre to centre and joined by a half-cylinder channel of 0.3 mm in diameter. The thickness of the A and Ā disks is 4 mm. The disk B is the core of the system and it can be changed to 0.10, 0.15, 0.20, and 0.30 mm nozzle diameter disks according with the size of the bulk layered crystals. The thickness of the B disk is 0.95 mm. For the exfoliation of layered crystals, the shear force generated by the solvent when the sample passes through the disk B, is the main phenomenon promoting the exfoliation [74].
At the end of the process, this method provides graphene dispersion in NMP, which has been used for its characterization and for the production of graphene powder.
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2.3 Graphene powder production
After the production of graphene dispersion, the powder is obtained by the following processes:
1. Removal of NMP and solvent exchange: NMP was removed using a rotavapor shown in Figure 18 at 88 °C and 5 mbar. Then, graphene was re-dispersed in ~300 mL DMSO. The resulting ink was transferred into aluminum petri dishes and kept in the freezer (T= -30 °C). The completely removal of NMP and the replacement with DMSO is necessary to facilitate the freezing process and the subsequent removal of the solvent by the freeze-drying.
Figure 18. Photo of rotavapor during the solvent evaporation of graphene-NMP dispersion. D.Farina@Istituto Italiano di Tecnologia
2. Freeze drying: the frozen petri dishes were then transferred to the lyophilize machines to remove DMSO employing Alpha 2-4LSC plus lyophilizer (Martin Christ, Figure 19). Freeze-drying from DMSO was performed at T = -10 °C and at P<0.1 mbar. This process lasts at least 50 h.
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At the end of the freeze-drying process, the powder was collected and used for the production of ABS and LLDPE composites following the procedures described in the following.
2.4 Composite production
2.4.1 Impregnation
The impregnation of the polymers consist in the following steps (see Figure 20):
1. Re-dispersion of the graphene powder in a solvent by performing sonication for about 30 minutes. In particular, the solvent used was isopropyl alcohol;
2. Addition of the polymers in form of pellets or powder to the dispersion;
3. About two hours of rotation at room temperature in a rotavapor followed by evaporation at ~55 °C and under vacuum (5-20 mbar).
Figure 20. Schematic representation of the impregnation process.
The impregnated polymers were then utilized to produce composites by three different methods: laboratory scale (using hot-press), semi-scalable (extrusion followed by hot-press) and scalable (extrusion followed by injection molding). These processes will be described in the follow paragraphs.
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2.4.2 Hot-press
Polymeric films employed to produce dog bones and to perform gas barrier measures were produced with Atlas™ Automatic, 8 Ton, Hydraulic Press (Specac) provided with heated platens, shown in Figure 21.
Figure 21. Photo of hot-press used to produce films
The samples were held in the pressing area between two heated plates prior to compression until the set temperature was reached (200 °C for ABS and 170 °C for LLDPE). Then a pressure of 1 T was applied and after 1 minute the sample started cooling. Once the films were cooled to room temperature, the pressure was removed.
2.4.3 Extrusion
Extrusion is a manufacturing method used to process most type of thermoplastic polymers. There are different types of extruders such as single-screw extruder and twin-screw extruder. The latter has two parallel screw that either rotate in the same direction (called co-rotating) or in opposite direction (called counter-rotating). Figure 22 shows a schematic representation of a co-rotating, twin-screw extruder.
Generally, the screws have the following three zones:
Feed zone: this zone feeds the polymer into the extruder;
Melting zone (also called compression zone): most of the polymer is melted in this section, and the channel depth gets progressively smaller;
Metering zone (also called the melt conveying zone): this zone melts the last particles and mixes to a uniform temperature and composition.
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Figure 22. Schematic representation of co-rotating, twin-screws extruder [94]. On the right, the top view cross section of the co-rotating twin-screws is shown [95].
In this work, the pristine polymers and the impregnated ones have been extruded in a co-rotating, twin-screw extruder (Bandera) with L:D ratio (i.e. the ratio between length and diameter of the screw) of 45 (Figure 23). Before the extrusion process, the impregnated polymers were dried in the oven for ~3 h at 50 °C under vacuum (<125 mbar) to drive out moisture. Then, the raw material, in the form of pellets or powder, was gravity fed from a hopper into the barrel of the extruder. The material melted due to a combination of mechanical energy generated by turning screws and heaters arranged along the barrel. Due to the screws, the molten polymer was then forced forward into the heated barrel and into a die which forms the polymer in the desiderate shape of constant cross section. In particular, the material was extruded into filaments with circular cross section, which was cooled and solidified as it through a water bath. Finally, the filament was cut into pellets utilizing a cutter to be used as precursor for further processing.
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The extruder employed in this work has five independent controlled heater zones with thermocouples for temperature control: four thermoregulation zones into the extrusion barrel wall (from Z1 to Z4) and one on the die (Z5). These allow to set a heating profile that gradually
increase the temperature of the barrel from the rear, where the plastic enters, to the front. In this way, the plastic pellets melt gradually as they are pushed through the barrel and the risk of overheating, which may cause degradation of the polymer, decreases. Table 3 reports the extrusion parameters employed for ABS and LLDPE.
Table 3. Profile temperature (T) and screw speed (RPM) for ABS and LLDPE polymers.
ABS LLDPE Zone T (°C) Screw speed (rpm) Zone T (°C) Screw speed (rpm) Z1 235 150 Z1 165 150 Z2 240 Z2 170 Z3 250 Z3 170 Z4 255 Z4 172 Z5 260 Z5 175
The melt processing parameters, i.e. temperature and screw rotation speed, of ABS and LLDPE have been set in order to limit their degradation and ensure a good filler dispersion. It was found that the extrusion at a profile temperature of 235-260 °C for ABS and 165-175 °C for LLDPE, with a screw speed of 150 rpm yields good extruded strings with a minimal residence time (~2 min).
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2.4.4 Injection molding
Figure 24. Scheme of injection molding machine.
Injection Molding is a manufacturing method most typically used in mass-production processes. In fact, this method allows to produce high volumes of the same object thanks to its reproducibility and it is ideal for processing thermoplastic polymers. Figure 24 shows a schematic representation of injection molding machine.
In particular, in this work the polymers were molded in dog bone shape with Babyplast 6/10P, which is one of the smallest fully hydraulic injection molding machines available (see Figure 25). It is the ideal machine for the production of micro parts from all types of thermoplastic materials.
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There are three thermoregulation zones: plastification (H1), chamber (H2) and nozzle (H3)
zones. Table 4 reports the temperatures employed in this thesis for the injection molding of ABS and LLDPE.
Table 4. Profile temperature for the injection molding of ABS and LLDPE.
ABS LLDPE Zone Temperature (°C) Zone Temperature (°C) H1 240 H1 160 H2 235 H2 155 H3 230 H3 150
The cycle of the injection molding process begins when the mold closes. The row material in the form pellets or powder is fed in to a hopper and then passed into the plastification zone where it melts. A piston pushes forward the melted material, which is collected in the injection chamber. When enough material is gathered (i.e. the volume of material that is used to fill the mold cavity and compensate for shrinkage), the nozzle injects the melted plastic at high pressure into the mold, which forms the polymer into the desired shape. Once the cavity of the mold is filled, a holding pressure was maintained to compensate for material shrinkage. The pressure is applied until the gate (i.e. cavity entrance) solidified. The gate is normally the first place to solidify and then no more material can enter the cavity. The material into the mold cools down and finally, ejectorspush the cooled product out of the machine and the injection molding process is complete. The time of cooling is reduced employing cooling lines circulating water from an external temperature controller. The temperature of chiller system was set at 15 °C.
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2.5 Characterization techniques
2.5.1 Atomic force microscopy (AFM)
In this work, AFM images were acquired with Bruker Innova AFM (Figure 26), operating in tapping mode at using silicon probes (frequency 300 kHz, spring constant 40 Nm-1) to measure the thickness of graphene flakes.
Figure 26. Bruker Innova atomic force microscopy [96].
The samples were prepared by drop-casting onto mica substrate the graphene dispersion diluted with NMP (1:100) from the original concentration. Then, the samples were washed in acetone and dried at ~150 °C for 30 min on a hot-plate. Thickness statistics were performed measuring ~100 flakes from AFM images and statistical analysis were fitted with log-normal distributions [97][98].
2.5.2 Transmission electron microscopy (TEM)
TEM images present in this work, were acquired by a JEOL JEM-1011 transmission electron microscope, operated at an acceleration voltage of 100 kV in order to measure the lateral size of graphene flakes. The samples were prepared by diluting the WJM dispersion (~10 gL-1) with NMP 1:100 and drop casting ~25 µL of the diluted dispersions onto ultrathin
C-film on holey carbon 400 mesh copper grids (Figure 27). The grids are stored under vacuum at room temperature to remove the solvent.
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Figure 27. Picture of holey carbon copper grid used as TEM support film. Adapted from [99].
Lateral size statistics were performed measuring ~100 flakes for each sample from TEM images using the software ImageJ. Then statistical analysis were fitted with log-normal distributions [97][98].
2.5.3 Scanning electron microscopy (SEM)
The SEM images of graphene-polymer composites were acquired using a JEOL JSM-6490LA (see Figure 28), operating at 10 kV acceleration voltage. The samples were cooled in liquid nitrogen and broken in order to analyze the morphology of the fractured surfaces. Then, the samples surfaces were coated with a 10 nm thick gold film in order to prevent the accumulation of electrostatic charge. The electron beam is thermoionically emitted from an electron gun composed of a cathode of tungsten filament.
Figure 28. “JEOL JSM-6490LA” scanning electron microscope.
In the present work, imaging operation was carried out with the secondary electrons in order to analyze the morphology of the sample’s fractures. The detected electrons are converted into an electric signal and processed by a computer to produce the final image displaying the topography of the surface.
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2.5.4 X-Ray diffraction (XRD)
XRD measurements were performed by using a PANalytical Empyrean X-ray diffractometer (see Figure 29b) equipped with a 1.8 kW Cu Kα ceramic X-ray tube, PIXcel3D 2X2 area detector and operating at 45 kV and 40 mA.
Figure 29. a) Schematic representation of X-ray diffraction process and b) photo of PANalalytical Empyrean X-ray diffractometer [100].
The x-rays from the source interact with the sample to produce diffraction patterns at the angles corresponding to specific crystal planes. The angle of diffraction is related to the specific crystal orientation of the sample by Bragg’s law as follows:
2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆 , (7)
where, d is the spacing between diffracting planes, θ is the incident angle, n is any integer, and λ is the wavelength of the X-ray beam as shown in Figure 29a. The diffraction pattern at specific angles obtained by XRD can be compared to the theoretical diffraction pattern calculated by the crystal planes to help identify the material.
In particular, the diffraction patterns were collected in Parallel-Beam (PB) geometry and symmetric reflection mode using a zero-diffraction silicon substrate over an angular range 2θ = 10-80, with a step size of 0.02° and a scan speed of 0.3°s-1. XRD data analysis was
carried out using High Score 4.5 software from PANalytical and the spectra were used to determinate the crystallization degree of LLDPE. The error was calculated as standard deviation of three value of the area under the XRD peak obtained using High Score software for each sample, since this is what most contributes to the error.
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2.5.5 Raman spectroscopy
Raman spectroscopy is based on the inelastic scattering of monochromatic light, usually from a laser source, and it is one of the most important tools in the characterization of the different carbon allotropes [101][102].
Photons of the laser light are absorbed by the sample and re-emitted with the same or different frequency. The large majority of the scattered light has the same frequency as the incident one and the process is an elastic scattering called Rayleigh scattering (see Figure 30). On the contrary, if the frequency of photons in monochromatic light changes upon interaction with a sample the scattering process is called inelastic. In this condition, the scattered photons have a reduced or increased frequency, a process known as Raman scattering [101]. There are two different Raman scattering processes that can be distinguished, i.e. Stokes scattering, where the frequency of scattered light is reduced with respect to the incident one, and Anti-Stokes scattering, where the resulting frequency of the scattered light is increased (see Figure 30) [103].
Figure 30. Energy-level diagram showing elastic/inelastic Rayleigh/Raman processes.
In particular, room-temperature Stokes Raman measurements were carried out for graphene samples utilizing a Renishaw inVia micro-Raman spectrometer with the 532 nm line of an Ar+ laser (Figure 31).
